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Furnaces fed with the natural draught of a chimney, and burning coke,
the earlier and most common design, are not economical, but for small casts
they are not to be despised. The best in this design are Carr's (fig. 62),
where the fire bars are placed below the bottom, leaving a space above,
through which most of the air passes. The melting is rapid, and the crucible
does not sink. The brick lining is carried on' a flange within the furnace
above the air-space, and a non-conducting backing of broken bricks fills the.
space between the lining and the outer casing of iron. Furnaces are built
to take one or more crucibles. Several furnaces can communicate with
flues leading to a common chimney, as in the ordinary brick furnaces.
Fig. 62.—The " Carr " Brass-melting Furnace
A, Solid lined furnace. B, Non-conducting space filled with broken brick, c, Chimney 30 to 35 ft. high.
D, Ash-pit. E, Pit for taking out ashes.
Improved designs of coke-fired furnaces in extensive use include pre-
heating of the metal, tilting of the crucible while in the furnace for pouring,
and the employment of artificial blast. In the first, the metal is placed in
a crucible or other annular vessel, above the melting crucible, where it is
warmed by the heat escaping from the fuel below, before it drops into the
lower crucible. The latter is not removed from the furnace, but both are
tilted for pouring. The preheater can be swung to one side when fresh
coke has to be charged. This design permits of the employment of larger
crucibles, and the attendant suffers less discomfort than when the crucible
with its charge has to be lifted out with tongs from above. The employment
of blast results in a great saving of coke, when, as is sometimes done, the
blast is warmed during its passage by the waste heat from the furnace. It
is possible in some of these designs to melt 1000 Ib. of brass in one charge.